Long Term Evolution (LTE) uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT)-spread OFDM (also referred to as single-carrier FDMA (SC-FDMA)) in the uplink. Figure (FIG. 1 illustrates one type of LTE downlink physical resource. The LTE downlink physical resource can be seen as a time-frequency grid, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval.
FIG. 2 illustrates the LTE time-domain structure. In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame 210 consisting of ten equally-sized subframes of length Tsubframe=1 ms, in the illustrated example embodiment.
In the LTE system, HARQ protocol is used to enhance transmission reliability. FIG. 3 illustrates HARQ operations in LTE. As depicted, when an initial transmission is not received correctly by the receiver, the receiver stores the received signal in a soft buffer and signals to the transmitter of such unsuccessful transmission. The transmitter can then retransmit the information (referred to as a transport block in LTE specifications) using the same channel coded bits or different channel coded bits. The receiver can then combine the retransmission signal with that stored in the soft buffer. Such combining of signals greatly enhances the reliability of the transmission.
In LTE, the ACK/NAK feedback is generally sent by the UE using one of two approaches depending on whether the UE is simultaneously transmitting a physical uplink shared channel (PUSCH):                If the UE is not transmitting a PUSCH at the same time, the ACK/NAK feedback is sent via a physical uplink control channel (PUCCH).        If the UE is transmitting a PUSCH simultaneously, the ACK/NAK feedback is sent via the PUSCH.        
The use of LTE carrier aggregation (CA), introduced in Rel-10 and enhanced in Rel-11, offers means to increase the peak data rates, system capacity and user experience by aggregating radio resources from multiple carriers that may reside in the same band or different bands and, for the case of inter-band TDD CA, may be configured with different UL/DL configurations. In Rel-12, carrier aggregation between TDD and FDD serving cells is introduced to support UE connecting to them simultaneously.
Up to now, the spectrum used by LTE is dedicated to LTE. This has the advantage that LTE system does not need to care about the coexistence issue and the spectrum efficiency can be maximized. However, the spectrum allocated to LTE is limited which cannot meet the ever increasing demand for larger throughput from applications/services. Therefore, a new study item was carried out in 3GPP on extending LTE to exploit unlicensed spectrum in addition to licensed spectrum.
The 3GPP work on “Licensed-Assisted Access” (LAA) intends to allow LTE equipment to also operate in the unlicensed radio spectrum. Candidate bands for LTE operation in unlicensed spectrum include 5 GHz, 3.5 GHz, etc. Unlicensed spectrum is used as a complement to the licensed spectrum or allows completely standalone operation.
FIG. 4 illustrates LAA in unlicensed spectrum using LTE carrier aggregation. LAA in unlicensed spectrum implies that a UE is connected to a PCell in the licensed band and one or more SCells in the unlicensed band. In this description, a secondary cell in unlicensed spectrum is referred to as an LAA secondary cell (LAA SCell). The LAA SCell may operate in DL-only mode or operate with both UL and DL traffic. Moreover, in future scenarios LTE nodes may operate in standalone mode in license-exempt channels without assistance from a licensed cell. Unlicensed spectrum can, by definition, be simultaneously used by multiple different technologies. Therefore, LAA as described above needs to consider coexistence with other systems such as IEEE 802.11 (Wi-Fi).
To coexist fairly with the Wi-Fi system, transmission on the SCell conforms to LBT protocols in order to avoid collisions and causing severe interference to on-going transmissions. This includes both performing LBT before commencing transmissions, and limiting the maximum duration of a single transmission burst. The maximum transmission burst duration is specified by country and region-specific regulations, for e.g., 4 ms in Japan and 13 ms according to EN 301.893.
In addition to standardization work for LAA in the 3GPP forum, other standard setting bodies are also working on related technologies. For instance, Multefire Alliance Forum is working on adding more procedures to the 3GPP LAA system to enable standalone operations of LTE in unlicensed spectrum.
In Rel-13, Licensed-Assisted Access (LAA) has attracted significant interest in extending the LTE carrier aggregation feature towards capturing the spectrum opportunities of unlicensed spectrum in the 5 GHz band. WLAN operating in the 5 GHz band nowadays already supports 80 MHz in the field and 160 MHz is to follow in Wave 2 deployment of IEEE 802.11ac. There are also other frequency bands, such as 3.5 GHz, where aggregation of more than one carrier on the same band is possible, in addition to the bands already widely in use for LTE. Enabling the utilization of at least similar bandwidths for LTE in combination with LAA as IEEE 802.11ac Wave 2 will support calls for extending the carrier aggregation framework to support more than 5 carriers. The extension of the CA framework beyond 5 carriers was approved to be one work item for LTE Rel-13. The objective is to support up to 32 carriers in both UL and DL.
To support up to 32 carriers in DL, the UCI feedback, e.g. HARQ-ACK bits will increase significantly. For each DL subframe, there is 1 or 2 HARQ-ACK bits per carrier depending on if spatial multiplexing is supported or not. Hence, for FDD, there can be up to 64 HARQ-ACK bits if there are 32 DL carriers. The number of HARQ-ACK bits for TDD is even larger to hundreds of bits depending on the TDD configuration. Therefore, new PUCCH format(s) supporting larger payload is necessary. Similarly, the piggyback of increased number of UCI bits also motivates the enhancements on UCI feedback on PUSCH.
Uplink transmissions are dynamically scheduled. For example, in each downlink subframe, the base station transmits control information about which terminals should transmit data to the eNB in subsequent subframes, and upon which resource blocks the data is transmitted. The uplink resource grid is comprised of data and uplink control information in the PUSCH, uplink control information in the PUCCH, and various reference signals such as demodulation reference signals (DMRS) and sounding reference signals (SRS) if the SRS is configured. DMRS are used for coherent demodulation of PUSCH and PUCCH data, whereas SRS is not associated with any data or control information but is generally used to estimate the uplink channel quality for purposes of frequency-selective scheduling.
FIG. 5 illustrates multiplexing data and control information in PUSCH. Specifically, an example uplink subframe with only data, DMRS and SRS is depicted. Note that UL DMRS and SRS are time-multiplexed into the UL subframe, and SRS are always transmitted in the last symbol of a normal UL subframe. The PUSCH DMRS is transmitted once every slot for subframes with normal cyclic prefix, and is located in the fourth and eleventh SC-FDMA symbols.
In LTE, control information can also be carried in the PUSCH instead of the PUCCH. Thus, data and control information can be multiplexed in the PUSCH. The control information can include e.g.:                Channel state information (CSI) which may further be comprised of channel quality indicator (CQI) and precoding matrix indicator (PMI) bits        Rank indicator (RI)        HARQ-ACK feedback        
According to LTE specifications TS 36.212, v.13.0.0:                The channel coded CSI bits are multiplexed with the channel coded data bits. The channel coded CSI bits are placed (i.e. assigned to resource elements) before the channel coded data bits. These bits are interleaved together into the available REs in the PUSCH. FIG. 6 illustrates multiplexing data and control information bits in PUSCH, where the CSI (CQI/PMI) bits occupy the first few rows of REs and data bits occupy most of the rest.        Coded RI bits are placed in PUSCH SCFDMA symbol #1, #5, #8 and #12 starting from the bottom. The REs occupied by the coded RI bits are avoided by the coded CSI and data bits.        Coded HARQ-ACK feedback bits are placed in PUSCH SCFDMA symbol #2, #4, #9 and #11 starting from the bottom. The REs occupied by the coded HARQ-ACK feedback bits are NOT avoided by the coded CSI and data bits. In fact, the LTE specifications TS 36.212 describes that coded HARQ-ACK feedback bits overwrite the REs that already contain coded data bits.        
A data and control information multiplexing procedure was designed in LTE Rel-8 when the envisioned HARQ-ACK feedback sizes were rather small, e.g., 1-2 bits. With such small HARQ-ACK feedback size, the overwriting of PUSCH data REs introduces negligible performance losses.